1. Field of the Invention
The present invention relates to a signal processing apparatus and method capable of suppressing the generation of burst error, and to a data recording/reproducing apparatus using the same.
2. Description of Related Art
The magnetic disk recorder represented as a data recording/reproducing apparatus has been more and more requested to have a capability of higher recording density, and the signal processing technology in the recording/reproducing system for supporting this request has also been developed toward the higher recording density capability.
In order to cope with the S/N ratio reduced by the intersymbol interference associated with high density recording, a partial response equalization system has been employed. For example, PRML (Partial Response with Maximum Likelihood detection) class 4 has been used to detect a signal sequence nearest to a reproduced signal by means of a known interference caused in a reproducing channel, and it is already utilized in the magnetic disk recorder.
FIG. 1 shows a flow of digital information reading processing in the conventional EEPRML (Extended Extended PRML). A signal 1 read from a head is equalized by a PR equalizer 2 into a signal 3. Then a decoded data sequence 5 that was actually recorded is estimated from the signal 3 by a maximum likelihood decoder 4. The estimated coded sequence 5 is supplied through a postcoder 6 to a 16/17 code demodulator 10, where it is decoded into an information data sequence 11. The information data sequence 11 undergoes error detection and correction in a Reed-Solomon decoder 12.
The most of the error sequences in the maximum likelihood decoder 4 have a short distance from a correct sequence. The error sequences with shorter distances from the correct sequence are examined by use of an error flow graph.
FIG. 2 is a schematic graph of error flow within a distance of 8 from the correct sequence in EEPRML. In each state (et-3 et-2 et-1 et), et represents error at time t. When et is 0, the corresponding bit has no error. Similarly when et is respectively + and −, the corresponding bits “0” and “1” have errors of “1” and “0”, respectively. The numbers attached on the arrows in the flow diagram indicate the distance from the correct sequence that increase with the transition of the corresponding errors. From FIG. 2, it will be understood that the error sequences from the maximum likelihood decoder 4 in EEPRML have errors of ± (+−+) (three consecutive errors), ± (+−+− . . . ) (four or more consecutive errors), and ± (+−+00+−+) in the order of shorter distance from the correct sequence. The frequency of actual error occurrence is affected not only by the distance from the correct sequence but by the mutual correlation between the error length and noise. The actual error is likely to occur in order of errors of three consecutive bits, one bit, two bits, five bits and four bits. Where, the error of four consecutive bits “0101 . . . ” is represented as “1010 . . . ” or vice versa.
Also by use of more advanced PRML or by slightly moving coefficients of partial response the frequency order is somewhat changed, but error tendency is not changed.
The short errors on the modulated codes in the maximum likelihood decoder 4 are expanded into burst error by the demodulator 10. If, for example, 16/17 modulation code is used, the worst expansion is 4 bytes. This corresponds to the worst value in the case where errors occur at the final bit of 16/17 code and are propagated to the next code word by the postcoder 6. This error expansion causes the correction ability of Reed-Solomon code to be reduced.